Bifocal lenses

ABSTRACT

Bifocal contact and intraocular lenses are described wherein a viewing area which has an extend generally corresponding to the maximum pupil area of the wearer, said viewing area having a central circular refractive zone having a first focal length corresponding to distance or reading vision, and a plurality of annular, concentric refractive zones which alternate between a second focal length corresponding to the other of reading and distance vision and said first focal length as they extend outwardly from said central zone, wherein the total number of said zones is at least 20. Preferably, at least one of the zones has a power which is more negative than a zone of the same character which is situated closer to the center of the lens.

This invention relates to bifocal lenses and in particular to bifocaland trifocal contact and intraocular lenses.

Various designs of bifocal contact lenses have been proposed but allrequire considerable amounts of chair-side time for satisfactory fittingof such lenses. Designs have been proposed in which the viewing areaconsists of a central circular zone, surrounded by concentric zones ofalternating near (reading) and distant vision. U.S. Pat. No. 4,890,913discloses such a lens. The underlying concept in the above patent is totry to ensure that at every pupil size the amount of light transmittedthrough the reading and distant vision zones is substantially equal. Ithas been found that with such lenses when correctly fitted, the weareris able to concentrate on the clearest image focused on the retina.

While the lenses described in the above U.S. patent have made a usefulcontribution to the art, and can reduce the amount of fitting timerequired, there are still some problems to be overcome. Lensesmanufactured in accordance with the above patent are most effective whenmanufactured as hard lenses, or from lens material which have a lowwater content. If, on the other hand, lenses are made in accordance withthe prior patent in a soft, high water content lens material, the zonesformed on a posterior lenticular surface tend to be “ironed out” by thepressure of the upper eyelid.

Another, unrelated problem of some prior art lenses, is that in certainlight conditions, reflections are seen from junctions between adjacentzones. This can have the effect of giving the appearance of rings arounda light source.

The present invention is, therefore, in one aspect directed to asolution to the above problems and the provision of a lens which can beused both as a contact lens, and as an intraocular lens from a varietyof conventional lens materials.

According to the present invention there is provided a contact orintraocular bifocal lens comprising a viewing area which has an extentgenerally corresponding to the maximum pupil area of the wearer, saidviewing area having a central circular refractive zone having a firstfocal length corresponding to either distance or reading vision, and aplurality of annular, concentric refractive zones which alternatebetween a second focal length corresponding to the other of reading anddistance vision and said first focal length as they extend outwardlyfrom said central zone, wherein the total number of zones is at least20. Preferably, the central circular zone has a focal lengthcorresponding to distance vision.

The provision of a very large number of alternating reading and distancezones in the viewing area is believed to be responsible for overcomingmany of the problems of the prior art lenses. One significant advantageis that by providing a large number of zones, it is much easier toensure that the relative areas of the distance and reading zones isclose to the preferred 50:50 ratio at all pupil sizes. As a consequence,the lenses provide good distance and reading vision in all lightingconditions. The number of zones is at least 20 and is preferably muchhigher, e.g. at least 30 and preferably at least 50. A typical number ofzones may be between 20 and 50, e.g., 25 to 40. There is no theoreticalupper limit but there will be a practical upper limit which isdetermined by the limitations of the equipment or system used to formvery large numbers of zones on the surface of a lens. Existing computercontrolled lathes should be capable of forming up to about 70˜80alternate zones on the lenticular surface of a standard contact lens, inwhich the major viewing area is about 6˜8 mm in diameter.

The alternating zones may be formed on the front surface of the lens,but are preferably formed on the rear surface of the lens. One advantageof forming the zones on the posterior surface of the lens is that anyimperfection in the surface is largely compensated by the tear fluidwhich will fill the space between the undulating profile forming thezones and the cornea.

Preferably, the maximum tear thickness between the anterior lens surfaceand the cornea is about 0.007 mm and may be as little as 0.003 mm.

In general, the maximum thickness of material removed between everyother zone to form the zones is about 10% of the total thickness of thelens.

Another, separate problem arises in fitting bifocal contact lenses. Thisrelates to the observation that eyes suffer from the fact that thecombined power of the cornea and the natural crystalline lens increasestowards the periphery. As a result, when the pupil dilates in lowlighting conditions, a patient with normal distance vision willexperience improved distance vision when wearing corrective lenses witha negative power of about 0.5 dioptres.

Another aspect of the present invention is based on the realisation thatimproved vision with bifocal lenses can be achieved by introducing aprogressive power variation in a multi-zonal concentric lens in whichthe negative power of one or both types of vision zones is increasedtowards the periphery of the viewing area of the lens. Such progressionin negative power should not exceed about 1 dioptre, preferably not morethan about 0.75 to 0.8 dioptre and, generally, will be in the range of0.25 to 0.75 dioptre. A power change of about −0.5 dioptre from centreto periphery being preferred.

According to a further aspect of the present invention, therefore, thereis provided a bifocal contact lens wherein, at least, the major viewingarea is divided into a plurality of near and distance vision zones whichare formed as annular zones of different radii from a point in theregion of the centre of the lens, each near vision zone being adjacentto a distance vision zone or intermediate vision zone and, wherein atleast one of the zones has a power which is more negative than a zone ofthe same character which is situated closer to the centre of the lens.Lenses in accordance with this aspect of the invention do notnecessarily have a larger number of alternating zones as describedabove. However, in a preferred embodiment, the number of zones is atleast 20 and often up to 70 or 80.

Preferably, at least one distance vision zone has a power which is morenegative than a distance vision zone situated closer to the centre ofthe lens. Generally, the distance vision zones have powers whichprogressively become more negative towards the periphery of the lensviewing area so that the zones towards the periphery have the highestdegree of negative power.

In general, there is no further increase in clarity by increasing thenegative power by more than about 1 dioptre. Normally, the increase innegative power will be limited to 0.75 to 0.8 dioptre. Usually, optimumbenefit is obtained where the negative power increase is in the regionof 0.5 to 0.6 dioptre. The maximum additional negative power at theperiphery is limited by the onset of reverse spherical aberration.

In the manufacture of lenses in accordance with both aspects of theinvention, the zones may be formed by machining the power or base curveof the lens. Preferably, however, the power is produced by machining ormoulding the power surface of the lens. In order to provide a smoothtransition between zones, it is preferable to make the zones at leastpartially aspherical, generally by making some or all of the near visionzones aspherical, so that aspherical lens surfaces merge smoothly intospherical lens surfaces with minimum discontinuity at the junctionbetween two zones. Lenses produced in accordance with this concept aredescribed in UK Patent Application No. 2,295,686 (de Carle), and thecontent of this prior UK specification is specifically incorporatedherein.

It may also be advantageous to manufacture lenses in accordance with theinvention in such a way that the optical centre of at least the majorviewing part of the lens is displaced by a small distance from thegeometric centre of the lens. The displacement is such that when thelenses are worn, the lens is offset nasally on the cornea. This conceptis described in PCT/WO 98/53360 (de Carle) and in European PatentApplication No. 0618474 (Menicon), and the subject-matter of thesespecifications is specifically incorporated herein.

In accordance with the second aspect of the invention, some or all ofthe zones may be provided by diffraction as described in U.S. Pat. No.4,637,697 (Freeman). Lenses in accordance with the first aspect of theinvention generally operate by refraction.

Various embodiments in accordance with the invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a section through the viewing area of a contact lens inaccordance with a first aspect of the invention,

FIG. 2 is a similar section through the viewing area of a secondembodiment of a contact lens in accordance with the first aspect of theinvention,

FIG. 3 is a plan view of a lens in accordance with the second aspect ofthe invention, and

FIG. 4 is a part section on the line X—X in FIG. 3.

Referring to FIG. 1, the lens comprises an anterior power surface (1)and a posterior base curve (2). The lens has a central circular distancevision zone (3) formed in the posterior of the surface (2) and thecentral circular region is surrounded by a first concentric circularreading zone (4), having a radius appropriate to give a readingaddition. A second concentric zone (5) for distance vision surrounds thefirst reading zone (4) having a refractive surface corresponding to thatof the central zone (3). Further alternating, reading and distancevision zones (4′) and (5′) extend outwardly from the central area (3) sothat, in total, the number of reading and distance vision zones in themajor viewing area total at least 20.

Referring to FIG. 2, this shows a currently preferred embodiment whichalso has an anterior surface (1) and a base curve (2). In a similarfashion to the embodiment of FIG. 1, there is a central circulardistance vision zone (3) and a series of closely spaced alternatingreading and distance vision zones (4) and (5). However, in the case ofthe FIG. 2 embodiment, the reading zones are formed by providing zoneswhich are flatter than the cornea, while the distance vision zones areformed from zones steeper than the cornea. The zones continuously mergeone with the other, and by providing a very large number of zones, theeffect of possible spurious reflections from the junction area (6) isreduced.

With the design shown in FIG. 2, half the reading addition effect isachieved by the portion which is flatter than the cornea (i.e. thereading zone) and the other half by the curve that is steeper than thecornea (i.e. the distance zone). In contrast, in the design shown inFIG. 1, the distance curvature approximately matches the corneal curveso that the entire reading effect is produced by the reading zones.

Lenses in accordance with the invention may be made from high watercontent soft lenses. Minus lenses often have a central thickness of theorder of 0.06 mm. To avoid the tendency for the inner surface of thelens to be pressed onto the cornea, thus eliminating most of the effectof the reading addition, as well as giving a poor optical effect, it isdesirable to reduce the thickness of the gap between the junction (6)and the cornea, where the lens is the greatest distance away from thecornea. Preferably, this distance should be less than about 0.007 mm,preferably between 0.003 and 0.006 or less.

It will be appreciated that by forming the reading correction on therear surface of the lens, the same profile of the rear surface can beused with a variety of front surface curvatures, depending on the basicdistance correction required. Thus, if the lenses in accordance with theinvention are manufactured by casting a monomer composition, the samerear surface mould half can be used with a number of different frontsurface mould halves, thereby reducing the inventory required.

Lenses in accordance with the invention may be manufactured by machiningor by moulding (casting). In the case of casting, the master moulds willbe produced by machining the desired profile into the master mould halffrom which the casting moulds are produced.

In the case of the embodiment of FIG. 2, the zones are convenientlyformed on the posterior lens surface by continuously changing the radiusof the cutting tool.

Referring to FIGS. 3 and 4, these show a simplified version of a lens inaccordance with a second aspect of the invention. The surface of acontact lens as shown in FIG. 3 has a major viewing area P consisting ofa plurality of concentric zones and an outer area Q. The area P is shownin section in FIG. 4 and comprises a diameter which will cover the pupilover the range of light conditions.

Although the major viewing area P is divided into six zones, including acircular central zone (13), normally the central viewing area will bedivided into a larger number of vision zones. As a minimum, there willbe at least two zones of each character, i.e. distance or reading visionbut, more commonly, the major viewing area will be divided into at leastseven zones, for example, eleven to nineteen zones. There is, however,no absolute limit to the number of zones employed. Generally, bettervision is experienced, the greater the number of zones as describedabove in relation to FIGS. 1 and 2.

As shown in FIG. 3, the central zone (13) is a distance vision zone andthis is surrounded by further distance vision zones (12) and (12 a) andby reading zones (11), (11 a) and (13 a). The outermost zone (12 b) mayalso be a distance vision zone but, generally, will not take much partin the correction of the patient's optical defect, except in very dimlight conditions.

The distance zone (13) at the centre of the lens does not have the samepower as the corresponding distance vision zones (12, 12 a) towards theperiphery of the major viewing area (P). Instead, the distance visionzones progressively increase in their negative power towards theperiphery. The increase may be progressive, for example, the first zonemay be 0.25 dioptres more negative than the central zone (13), while thethird zone may be 0.5 dioptres more negative, and the final zone (12 b)may be a further 0.25 dioptres more negative so that it is 0.75 dioptresmore negative than the central zone. The increase in negative powertowards the periphery may, however, not necessarily be even, and severaldistance vision zones towards the periphery may have similar negativepowers.

The reading zones may also increase in their negative power towards theperiphery by the same or different degrees of negative power. Eachreading zone may, for example, have the same reading addition inrelationship to an adjacent distance vision zone so that effectively thereading zones become more negative towards the periphery. Alternativelyin a lens complex design, the reading zones may have the same readingaddition in relationship to the innermost distance vision zone.

FIG. 4 shows diagrammatically how the zones are cut in order to formalternate distance and reading refractive zones. These may be formeddirectly using a high precision computer-controlled lathe or,alternatively, the powers are cut onto the surface of the mould fromwhich the lenses are produced.

Although, in the specific embodiment described above, the central zoneis designed for distance vision and the adjoining annular zone forreading, it will be appreciated that the situation may be reversed.

Preferably, the lenses manufactured in accordance with this inventionwill be produced from material of the hydrated hydrophilic polymer type,so that the lenses are formed by machining or moulding a xerogel and theresulting products are then swollen by hydration in isotonic saline toproduce the final lenses.

Although the invention has been described in connection with itsapplication to the manufacture of bifocal lenses, it will be appreciatedthat the invention is also applicable to multi-focal lenses in which thevision zones may be divided into alternate near, intermediate anddistance vision zones.

The annular zones and/or the basic power of the lens may be provided byrefraction. However, the invention includes the use of diffractive ringsto provide some or all of the power of the near or distance visionzones. In this connection, reference is made to U.S. Pat. No. 4,637,697(Freeman) for details of the formation of diffractive power on a contactlens having basic refractive power.

What is claimed is:
 1. A bifocal contact lens having a viewing area generally corresponding to the maximum pupil area of the wearer, said viewing area having a central circular refractive zone having a first focal length corresponding to distance vision or reading vision and a plurality of annular refractive zones which alternate between a second focal length and said first focal length corresponding to the other of reading and distance vision as they extend outwardly from said central zone, wherein at each pupil size between a minimum and a maximum the relative areas of the distance and reading zones are between 40:60 and 60:40, the total number of zones comprises at least 30, the zones are alternately flatter or steeper than the cornea and there is a progressive increase in negative power in at least some of the zones of the same character towards the periphery of the viewing area.
 2. A lens according to claim 1 wherein the alternating zones are formed on the posterior lens surface.
 3. A lens according to claim 2 wherein the maximum thickness of material removed between every other zone to form the zones is about 10% of the total thickness of the lens.
 4. A lens according to claim 1 wherein the maximum tear thickness between the posterior lens surface and the cornea is about 0.007 mm.
 5. A lens according to claim 1 wherein the zones are formed on the posterior lens surface by continuously changing the radius of the cutting tool.
 6. A lens according to claim 1 wherein at least some of the distance vision zones have powers which progressively become more negative towards the periphery of the lens viewing area.
 7. A lens according to claim 1 wherein at least some of the near vision zones have powers which progressively become more negative towards the periphery of the lens viewing area.
 8. A lens according to claim 1 in which zones situated in the region of the periphery of the viewing area are more negative by a power of up to 0.75 dioptres than a zone of the same character situated close to the lens centre.
 9. A lens according to claim 8 wherein zones situated in the region of the periphery of the viewing area are more negative by at least 0.25 dioptres than a zone of the same character situated close to the lens centre.
 10. A lens according to claim 1 wherein at least one of the near or reading vision zones is aspherical.
 11. A lens according to claim 1 wherein the surface area of the near (reading) vision zones is substantially equal to the surface area of the distance vision zones.
 12. A lens according to claim 1 wherein at least some of the annular zones in a central region of the lens are formed concentrically about an optical centre which is spaced from the geometric centre of the lens so that in use the optical centre is offset nasally.
 13. A lens according to claim 12 wherein the offset is about 0.3 to 2 mm.
 14. A bifocal contact lens comprising a disc-like member having a concave surface and a viewing area on said surface, said viewing area including a central zone providing a distance or near vision correction, a plurality of concentric annular zones surrounding said central zone each providing, in alternation with increasing distance from said central zone, distance or near vision correction, the total number of zones being at least 30, the total area of the near vision zones being substantially equal to the total area of the distance vision zones, and said zones comprising depressions in said surface of generally equal depth and of a maximum depth of around 0.006 mm. 